Reddit reviews Gemstones of the World: Newly Revised Fifth Edition

Actually the padparadscha (Sinhalese: lotus flower) is just another variety of fancy sapphire. Red corundum are rubies, everything else in the corundum species is a sapphire.

Also there's no definite demarcation of exactly how red a corundum needs to be before it's called a ruby.

Source: Gemstones of the World: Newly Revised Fifth Edition

After some digging, I came across a number of books. I'll come back and leave some notes after I've read them.

• The Complete Jewelry Making Course by Jinks McGrath (link)
  
• Gemstones of the World by Walter Schumann (link)
  
• The Jeweler's Directory of Gemstones by Judith Crowe (link)
  
• Gemstone Setting by Anastasia Young (link)

Thanks for the recommendation! I just bought this book:
Gemstones of the World: Newly Revised Fifth Edition https://www.amazon.com/dp/1454909536/ref=cm_sw_r_cp_api_pPGIybE1G50X2

Given that I didn't know an Amethyst was a quartz, I figured it's a good place to start. The book you recommended seems like a logical next step once I've upped my technical knowledge of gemstones themselves!